1. Field of the Invention
The present invention relates to a catalyst for preparing a carbon nanotube having desired apparent density by controlling the adding amount of co-precipitating agent in the process of preparing a catalyst in order to obtain a catalyst having a minimized particle size. More specifically, this invention relates to a catalyst for preparing carbon nanotube having desired apparent density based upon the reverse-correlation between the amount of co-precipitating agent used in the process of preparing catalyst and the apparent density of catalyst. The carbon nanotube prepared by the catalyst having low apparent density shows excellent electrical conductivity and highly uniformed dispersion in the polymer/carbon nanotube composite.
2. Description of Prior Art
Carbon nanotube has a hexagonal honey comb structure in which one carbon atom is bonded with 3 adjacent carbon atoms through sp2-hybridization. Further, it has around 5 to 100 nm of diameter and high aspect ratio as well as a graphite structure which is composed of carbon atoms. Therefore, such a chemical structure of carbon nanotube results in high mechanical strength, excellent electrical and thermal conductivity. Due to these superior physical properties of carbon nanotube, carbon nanotube has been expected as future alternative material by replacing conventional carbon black or carbon fiber. Further, the industrial application of carbon nanotube has been also expected to be expanded.
As the factors for controlling the diameter and aspect ratio of carbon nanotube, the metal element of catalyst, the composition ratio of metal element, the reaction temperature and/or the kind of gas source can be exemplified. Among the factors indicated above, the metal element of catalyst and the composition ratio of metal element can be considered as the most important factor in determining the structure of carbon nanotube. In general, the diameter of carbon nanotube can be determined by the particle size of catalyst. In other words, the diameter of carbon nanotube becomes smaller when the particle size of catalyst becomes smaller.
Even though the catalyst for preparing carbon nanotube can be prepared in various methods, the co-precipitation method and the impregnation method can be widely used for preparing carbon nanotube by chemical vapor deposition method. Generally, the co-precipitation method for preparing catalyst has been preferably used for its convenience, because the porous supported material is not required in this method.
The co-precipitation method comprises the following steps; i) dissolving the metal salt in the water; ii) inducing the precipitation of metal salt by adding co-precipitating agent along with adjusting the pH of the solution; and iii) filtering and drying the obtained precipitates. Finally, the catalyst precursor has been obtained as a powder type. After thermal oxidation or reduction of catalyst precursor, the catalyst for preparing carbon nanotube can be obtained.
The impregnation method requires porous supported material, where metal salt can be impregnated. After mixing metal salt and porous supported material in the aqueous solution, a slurry type of catalyst precursor has been obtained. Then, catalyst precursor has been obtained by filtering and drying the slurry type of catalyst. After thermal oxidation or reduction of catalyst precursor, the catalyst for preparing carbon nanotube can be obtained.
As described above, the catalyst for preparing carbon nanotube according to co-precipitation method or impregnation method can be obtained as a form of fine powder. The particle size of catalyst can be in the range of 100 nm to 500 micrometer, ordinarily 10 micrometer to 100 micrometer.
Since it has a limitation for minimizing the particle size of catalyst according to co-precipitation method, the control of apparent density of catalyst can be an alternative to obtain a catalyst having a minimized particle size. If carbon nanotube has been prepared using a catalyst having a minimized particle size, the carbon nanotube can show excellent physical properties. For example, if the polymer/carbon nanotube composite is prepared using said catalyst, the excellent uniform dispersion and the high electrical conductivity of polymer/carbon nanotube composite can be accomplished.
However, it has not been tried to control the apparent density of catalyst to obtain a catalyst having a minimized particle size. Followings are preparation method of catalyst for preparing carbon nanotube according to co-precipitation method.
According to the paper, ‘Journal of the Korean Ceramic Society Vol. 36, No. 5, 504˜512, 1999’, about 2.2 equivalents of co-precipitating agent as base were used per 1 equivalent of total metals, when preparing two elements of metal catalyst of iron and copper/iron and nickel by a co-precipitation method.
In Korean Early Publication No. 2007-84180 ‘Catalyst for producing carbon nanotubes by means of the decomposition of gaseous carbon compounds on a heterogeneous catalyst’, it has been disclosed that a catalyst for preparing carbon nanotube is prepared by the co-precipitation method in which ammonium carbonate or sodium hydroxide has been added to the metal salt solution comprising Co, Mn, Mo, Al and/or Mg salt. Further, it has been disclosed that less than 2.4 equivalents of co-precipitating agent as base are added per 1 equivalent of total metals. However, it has not been disclosed that apparent density of catalyst can be controlled by adjusting the amount of co-precipitating agent.
In Korean Early Publication No. 2007-86611 ‘Method for synthesizing a supported catalyst for the production of carbon nanotubes’, it has been disclosed that a catalyst for preparing carbon nanotube is prepared by an impregnation method using aluminum hydroxide as supported material. The process for preparing a catalyst comprising i) preparing a paste by mixing aluminum hydroxide having less than 80 μm of particle size in the iron and cobalt salt aqueous solution; ii) selecting and obtaining the catalyst particle having less than 70 μm of particle size after drying and sieving the paste. However, in case of an impregnation method, the particle size of carrier will determine the particle size of catalyst. Therefore, it is hard to control the particle size and apparent density of catalyst. Further, it cannot be considered as an economical method for producing the catalyst in a large scale, because the selection of the catalyst particle having small particle size by sieve requires an inconvenience of process.
Therefore, the amount of co-precipitating agent used in the course of preparing a catalyst for carbon nanotube has been less than 2.5 equivalents as to total metal amounts, because the increase of co-precipitating agent causes the increase of preparation cost of catalyst. Of course, it has not been tried to control the apparent density of catalyst by controlling the amount of co-precipitating agent.
The inventors of the present invention have researched the process for controlling the apparent density of catalyst for preparing a carbon nanotube. We found that the increase of the amount of co-precipitating agent used in the process of a co-precipitation method results in the reduction of apparent density of catalyst. Further, the reduction of apparent density of catalyst causes to minimize the particle size of catalyst. Of course, the catalytic yields of said catalyst having a minimized particle size also can be enhanced. Further, if the carbon nanotube is prepared by said catalyst having a minimized particle size, polymer/carbon nanotube composite can show high electrical conductivity, because the carbon nanotube prepared by said catalyst can be uniformly dispersed in the polymer/carbon nanotube composite.
Finally, the inventors of the present invention developed a method for preparing a catalyst having a minimized particle size by controlling the apparent density of catalyst, which can be accomplished by the adjustment of added amount of co-precipitating agent.